1. Field of the Invention
The present invention relates to a composition containing an aqueous dispersion of at least one organic polyhydroxy compound as a binder component and at least one monomeric diisocyanate dispersed in the dispersion of the polyhydroxy compound as a curing component, which may be used to make polyurethane coatings. A major advantage of the present invention is that the composition may be used to prepared virtually solvent-free. The invention also relates to applying polyurethane coatings to substrates using the composition.
2. Description of the Related Art
Waterborne coating systems are becoming increasingly important for both economic and ecological reasons. In chemically crosslinking two component polyurethane coatings, compounds which contain free isocyanate groups ("polyisocyanates") act as a curing agent for a binder, which is generally an organic polymer having NCO-reactive groups. Examples of binders include polyacrylatepolyols, polyetherpolyols and polyesterpolyols. These coating systems are of great importance because of the excellent properties of the finishes they produce. Until recently, organic solvents were indispensable in making these coatings. The use of water appeared not to be possible for a long time, since isocyanate groups are known to react, not only, as desired, with the organically attached NCO-reactive groups (such as hydroxyl groups) of the binder, but also, undesirably, with water. This gives rise to urea structures and produces carbon dioxide. The undesired reaction with water leads to (1) a reduced crosslink density in the cured finish and (2) foaming as a consequence of the carbon dioxide evolution, both of which are disadvantageous with respect to producing the coating as well as to the optical properties of the finishes. Altogether, such waterborne systems appeared to be impractical because of too short a pot life (or processing time) and unsatisfactory properties of the finishes.
The hydrolysis of isocyanate groups can be prevented by blocking (or capping) them. For this, the free polyisocyanates are reacted with blocking agents, such as secondary or tertiary alcohols; lactams (for example .epsilon.-caprolactam); or oximes (for example methyl ethyl ketoxime). The blocked polyisocyanates are stable at room temperature and at moderately elevated temperatures and can be mixed with the binder to form waterborne one component coatings. At the curing temperatures of the coatings, the blocking agent is detached, and the free isocyanate groups then react with the reactive groups, usually hydroxyl groups, of the binder to crosslink the chains. It is true that blocked isocyanates avoid the problems associated with the hydrolysis of the isocyanate groups. However, the blocking process adds an additional processing step and requires appreciably higher curing temperatures. Also, curing blocked isocyanates releases a blocking agent which may not necessarily be dischargeable into the environment.
There was, therefore, still a need for a waterborne two component polyurethane coating system featuring a curing agent containing free isocyanate groups. According to DE 27 08 422 and DE 35 29 249, organic di or polyisocyanates are added to polymer dispersions to improve the property spectrum. However, the dispersed polymers are not organic polyhydroxy compounds, as customary in polyurethane chemistry as reaction partners for polyisocyanates, but, instead, substances which are inert toward isocyanate groups. The property-improving effect is therefore based, not on chemical crosslinking, but on the dispersed polymer becoming sheathed with the polyurea compound formed from the reaction of the isocyanate with water.
Chemically crosslinking aqueous two component polyurethane coatings containing free polyisocyanates as the curing agent for the binder were apparently described for the first time in EP 0 358 979. According to this reference, polyhydroxyacrylates used as binder component are capable of emulsifying certain polyisocyanates having free isocyanate groups, known as coating polyisocyanates. The resulting aqueous two component system cures to crosslinked films. The coating polyisocyanates are oligomeric derivatives, containing biuret, urethane, urethdione and/or isocyanurate groups, of readily available monomeric or simple diisocyanates, especially of hexamethylene diisocyanate (HDI), isophorone diisocyanate (IPDI) and/or bis(isocyanatocyclohexyl)methane (H.sub.12 MDI or, in short, HMDI). The coating polyisocyanates used according to EP 0 358 979 have a 23.degree. C. viscosity of up to 1000 mpa.s and an average NCO functionality of 2.2 to 5. Monomeric diisocyanates as sole curing agents are therefore ruled out because of their lower viscosity as well as because of their bifunctionality. But they are even evidently undesirable as ingredients of the mixture, since, according to EP 0 358 979, they are separated from the coating polyisocyanates in a known manner, preferably by distillation, after the coating polyisocyanates have been produced. The coating polyisocyanates, like the aforementioned blocked polyisocyanates, have to be prepared using a separate process step and, what is more, owing to their higher molecular weight, have to be diluted with organic solvents to a suitable application viscosity or rendered self-dispersible via the incorporation of ionic- or nonionic-hydrophilic centers.
Other publications deal mainly with the widening of the field of use for emulsifying binder components and the variation, including through a nonionic-hydrophilic modification by alkoxylation, of the polyisocyanate curing agent components, especially with the goal of lengthening the pot life and of improving the property spectrum of the coatings. Hydrophilically modified polyisocyanates are, however, problematic for use as a curing agent component, since the cured finishes may be water-sensitive because of the hydrophilic groups which are still present. Examples of binder variations and/or the use of hydrophilic modified polyisocyanates are found in DE 41 01 696, 41 35 571, 42 26 270, 43 17 791, 43 22 242 and 43 26 670 and also in EP 0 469 389, 0 496 205, 0 542 085, 0 543 228, 0 562 436 and 0 583 728. Specifically, the preparation of hydrophilic modified polyisocyanates for use in aqueously disperse two component polyurethane systems is discussed in EP 0 206 059, 0 516 277, 0 540 985, 0 548 669, 0 645 410, 0 680 983 and 0 703 255, among others.
The isocyanate curing agents which are predominantly used and produced on an industrial scale for aqueous two component polyurethane systems generally have an average NCO functionality which is within the range of 2.2 to 5, as specified in EP 0 358 979. They are typically used to produce waterborne clear coating systems having a solids content of 40 to 50% by weight. However, these coating systems have the disadvantage of customarily containing, in the ready-to-use formulation, organic solvents in minimum quantities of 10 to 20% by weight, since this is necessary for optimum processing and to achieve the desired coating property profile. Aqueously disperse two component polyurethane coating systems comprising monomeric diisocyanates and an amount of organic solvents &lt;1% by weight, based on the solids content, have hitherto not been explicitly described, nor are they are commercially available. It is true that the use of monomeric diisocyanates as curing agents is explicitly or implicitly mentioned in a number of patent documents (for example in EP 0 562 436, EP 0 583 728, DE 43 26 670 and DE 42 26 270), but the examples always feature an organic solvent in amounts of 10 to 20% by weight in the ready-to-use formulation.